You will be given a definition of a task first, then some input of the task.
In this task, you are given a context and four options. Each option is a suggested ending for the context. You should read the context and pick the best ending for the context. Please answer with "A", "B", "C", and "D". 

[header] How to build a natural looking waterfall with a filtrific vanishing water system [title] Determine the path of the water feature. [step] When mapping out the path of your water feature, consider bends in the stream and waterfall drops. A natural looking stream channel should appear " ageless, " as if it has always been there. <sep> (A) [title] Figure out how thick the waterfall is. [step] Decide if it is a horizontal waterfall or an uphill stream, as natural looking streams typically come apart easier. (B) This opens up the drop space for the waterfall. [substeps] Your waterfall should look like a river channel with a naturally flowing stream, and be easily identified from sight. (C) [title] Determine where the current is pulling you. [step] By determining how the current will drag you, you determine how likely you are to build your waterfall. (D) To best accomplish this, choose a location with a slope. If you don't have much of a slope to work with, as an alternative you can choose to create water flowing from a boulder or rock grouping.
Output:
D